The general aim of the proposed experiments is to study ion permeation through a specific channel system--acetylcholine-activated channels at the neuromuscular junction. Single muscle fibers in the cutaneous pectoris muscle from Rana pipiens will be voltage-clamped using two intracellular microelectrodes. The agonist will be applied to the neuromuscular junction iontophoretically. The extracellular bathing solution will contain different cations, and the reversal potential and single channel properties will be determined for the different solutions. One specific aim will be to investigate how ion permeation is affected by the specific agonist and by the particular ions in the bathing solution. These results will be analyzed using a two-barrier, one-well Eyring rate theory model for the channel. A second specific aim will be to study the selectivity of the channel as a function of the particular agonist or the particular solvent.